Raman scattering and nonlinear refractive index measurements of optical glasses

Abstract

The absolute Stokes-Raman scattering cross section is measured for optical glasses of varying composition, from which the nuclear contribution to the nonlinear polarizability is determined. The nonlinearities increase progressively from fluoroberyllate compositions to fluorophosphate to phosphate to silicate and finally to heavy-metal oxide glasses. Although values of the total nonlinear polarizability vary by nearly two orders-of-magnitude it is determined that the ratio of the nuclear to the electronic contribution is nearly constant and is independent of the glass composition. The electrostrictive contribution to the nonlinear polarizability is also calculated using previously measured elasto-optic constants. Correlations in the atomic structure were determined by measuring the position of the lowest-frequency Raman scattering peak and it is found that the correlation length for pure one-component glasses is larger than multi-component glasses. Examination of the low-frequency scattering region shows that there is an 0xcess light-scattering below 30 cm −1 in all glasses tested. This excess scattering in LaSF-7 was fitted between 2 to 20 cm −1 to a Lorentzian centered at zero frequency with a halfwidth of 2.5 ± 1 cm −1, while the fit in pure silica between 5 and 30 cm −1 yields a halfwidth of 8 ± 3 cm −1.